I attempted marbling on fabric for the first time this weekend with the Jacquard marbling kit and some fabrics worked beautifully, but one fabric, a smooth cotton blend, wouldn't have the paint stick. When I peeled it back, the paint and size would just bead on the surface and drip away.I could not find any information as to why this may happen. Do I need more Alum? Or do some fabrics just not work?

Ahhh. This fabric must have been treated with something. It could have been waterproofing. I have seen this especially on things like tote bags that have been treated to have water bead up on them.

I haven't seen a fabric that you could not marble, except for these fabrics that are specially treated. Sometimes you can get these treatments out by washing, but that doesn't always work. Using pfd fabric (prepared for dyeing) will ensure that you have something that will work. Sizing like starch and others are not great for marbling either. Ideally you want a raw untreated fiber that is at least a little absorbent.

It can really depend on the type of fabric how well it sticks as well. Alum is always good, but I have seen color stick great to certain cottons especially PED(prepared for dying) and even very smooth polyester with no alum. It can be a bit of mystery. It is better to alum the fabric if there are any doubts.